1. Field of the Invention
The present invention relates generally to the technical field of mobile communications, and specifically to user apparatuses, radio base stations, and methods.
2. Description of the Related Art
A service area of a mobile communications system includes regions (called cells) covered by abase station. The cells are classified into macro cells and micro cells. Typically, a region of the macro cell is large (a coverage is large), and transmission power of the base station is also large. On the other hand, a region of the micro cell is small (a coverage is small), and transmission power of a base station is also small. Of the micro cells, a cell with even a smaller coverage is called a pico cell. As an example, the pico cell is arranged at a location where traffic tends to concentrate, and complements the macro cell or the micro cell.
FIG. 1 shows how the macro cell and the pico cell neighbor. For convenience of explanations, a base station for the macro cell is referred to as BS1, while a base station for the pico cell is referred to as BS2. A downlink (DL) signal transmitted from the BS1, which attenuates with distance, reaches relatively far as it is transmitted with high power. As shown, a solid line extending from the BS1 schematically shows received power (power at the time of reception at a user apparatus) of a DL signal. A downlink (DL) signal transmitted from the base station BS2 for the pico cell, which also attenuates with distance, reaches relatively near only as it is transmitted with low power. As shown, a solid line extending from the BS2 schematically shows received power (power at the time of reception at a user apparatus) of a DL signal.
If transmission power of BS1 is the same as transmission power of BS2, the DL received signal power from the base station BS1 and the DL received signal power from the base station BS2 become theoretically equal at an intermediate point B. However, in the present example, the transmission powers of the respective base stations differ, so that the DL received signal powers of the respective base stations become equal at a point A shown (a point which is closer to BS2 than to the intermediate point B). This means that quality of a DL received signal from the base station BS1 is better in a section from the base station BS1 to the point A, in which section the base station BS1 is suitable for downlink communications. Moreover, in a section from the point A to the base station BS2, quality of a DL received signal from the base station BS2 becomes better, so that, in that section, the base station BS2 becomes a base station suitable for downlink communications. Typically, transmission power of the base station BS1 for the macro cell is greater than transmission power of the base station BS2 for the pico cell, so that a distance from the base station BS1 to the point A is greater than a distance from the base station BS2 to the point A.
On the other hand, a path loss or propagation loss is related to the difference between the transmission power and the received power, so that it depends, not on power at the time of transmitting, but on a distance between the base station and user apparatus. The greater the distance, the larger the path loss. In other words, the inverse of the path loss ((path loss)−1) becomes small depending on the distance from a base station. Thus, as shown, the inverse of the path loss with respect to the base station BS1 and the inverse of the path loss with respect to the base station BS2 become equal at the intermediate point B. Whether quality of an uplink signal from a mobile station to a base station is good is related to the magnitude of the path loss (or the inverse thereof). Therefore, from the base station BS1 to the point B, quality of an uplink signal to the base station BS1 becomes better, so that in that section the base station BS1 is suitable for uplink communications. Moreover, from the base station BS2 to the point B, quality of the uplink signal to the base station BS2 becomes better, so that in that section the base station BS2 is suitable for uplink communications.
In this way, when transmission powers of neighboring base stations differ, a section (shaded portion in FIG. 1) occurs such that an optimal base station differs between downlink communications and uplink communications. If a user is within the section of the shaded portion, the following three methods of connection are possible:
(1) Connection is Made to the BS1 for Both Uplink and Downlink.
FIG. 2A shows how to connect to the BS1 for both uplink and downlink. In this case, downlink communications may be conducted well. On the other hand, for uplink communications, the user apparatus needs to transmit a signal with high power in order to ensure that a signal arrives at the base station BS1 via a relatively long distance. However, there is a concern that this may cause high interference power to the base station BS2, which is close in distance; and
(2) Connection is Made to the BS2 for Both Uplink and Downlink.
Contrary to FIG. 2A, FIG. 2B shows how connection is made to the BS2 for both uplink and downlink. In this case, uplink communications may be conducted well. However, for downlink communications, there is a concern that the user apparatus may receive relatively large interference from the BS1 since received power of a signal from the BS1 is greater than received power of a signal from the BS2.
In this way, when the same base station is selected for both downlink communications and uplink communications, either of the downlink signal quality and the uplink signal quality may deteriorate. Moreover, there is a concern for deterioration of system capacity and wasting of radio resources due to interference received by a base station or a user apparatus becoming large,
(3) Connection is Made Separately for Uplink and Downlink.
FIG. 3 shows how different base stations are selected between downlink data transmission and uplink data transmission when user equipment (UE) is in the section of the shaded portion of FIG. 1 (for this type of system, see Non-patent document 1, for example). More specifically, the user equipment UE receives downlink data from the base station BS1 for the macro cell and transmits uplink data to the base station BS2 for the pico cell. In this way, the above-described concerns may be dealt with.
Now, certain information needs to be fed back in uplink for downlink data transmission. Certain information also needs to be fed back in downlink for uplink data transmission. For both uplink and downlink, a representative example of information requiring feedback is acknowledgement information, which shows acknowledgement (ACK) or non-acknowledgement to received data. Moreover, channel quality indicator (CQI), which shows how good downlink channel state is, is transmitted in uplink. For the present method, feedback information on downlink data is transmitted from the user equipment UE to the base station BS1 for the macro cell. The feedback information on the uplink data is transmitted from the base station BS2 for the pico cell to the user equipment UE.
Thus, with this method, there is again a concern for problems described in (1) and (2) with respect to transmission of feedback information. In other words, the base station BS2 for the pico cell ends up receiving large interference due to feedback conducted in uplink. Moreover, feedback conducted in downlink ends up receiving large interference from the macro cell.
Non-patent document 1 3GPP, C.S0084-001-0, “Physical layer for Ultra Mobile Broadband (UMB) air interface specification,” August 2007.